For offset printing, the plates which have been used prevailingly have been not only a presensitized plate using an aluminum plate as a substrate (hereinafter referred to as "a PS plate") but also a presensitized plate using paper or a plastic film, e.g. a polyester film, as a substrate (hereinafter referred to as "a paper plate" or "a film plate" respectively). In particular, the use of a low-priced paper or film plate has markedly increased in frequency because of a recent tendency to diversify the purpose of printing and require a small number of copies in printing. Under these circumstances, reduction in time required for replacement work of machine plates has become a great problem.
In a case where a printing of a small number of copies, such as 300 copies, was required, although the printing time from the start to the finish of printing was about 5 minutes, the time required to replace of machine plates was at least 5 minutes in the past. In other words, in some cases the time for attachment of a machine plate was longer than the net printing time. Therefore, many devices which enable a reduction in working time for replacement of machine plates have been proposed in recent years. For instance, such devices are disclosed in Tokkai Sho 59-1262, Tokkai Hei 3-93546, Tokkai Hei 6-328669, Tokkai Hei 8-39779 (The term "Tokkai" as used herein means an "unexamined published Japanese patent application"), Jikkai Hei 3-19037, Jikkai Hei 3-45133, Jikkai Hei 3-45134 (The term "Jikkai" as used herein means an "unexamined published Japanese utility model application"), and Jikko Hei 6-11782 (The term "Jikko" as used herein means an "examined Japanese utility model publication").
A machine plate mounted on a plate cylinder, which is equipped with a device such as described in those gazettes, has a plurality of circular holes for fastening purpose (basic holes) which are punched in a row at each end situated in the length direction thereof, and each of those devices has machine plate fastening pins (basic pins) which are fitted in the desired positions of a PS plate lock-up device placed in the cut-off part of a plate cylinder. Thus, the machine plate registering and fastening operation is performed by inserting the basic pins in the basic holes.
Each of those devices requires a PS Plate lock-up device or/and a plate cylinder to undergo certain direct procedures for fixing machine plate fastening pins (basic pins) thereto. However, it is difficult to perform accurate work at the job site. In attaching such a device to a printing machine which has already been in operation, therefore, it is necessary to detach the parts to be worked on from the printing machine and to work on them with a NC machine in order to secure printing accuracy; as a result, the operations at the job site increase in complexity.
For instance, the machine plate attaching device described in Japanese Jikko Hei 6-11782, has basic pins which are provided so as to project out from the upper surface of clamp plates of a PS plate lock-up device and which are inserted in basic holes from the lower side of the machine plate. Further, that attaching device is fitted with plates which are designed so as to cover the basic pin zone and press a machine plate to the upper surface of the clamp plates of the PS plate lock-up device (and so these plates are abbreviated as "presser plates" hereinafter). Although such a device improves the working efficiency in attaching a machine plate to a plate cylinder when the machine plate is made of paper, workers are liable to be injured by presser plates. In addition, when the machine plate to be attached is made of a material having high elasticity, such as a plastic or aluminum sheet, such a device has a disadvantage in that, as the machine plate is apt to come up and get out of basic pins due to the reaction force the machine plate has when it is curved along the peripheral direction of a plate cylinder, it has a tendency to slide off the plate cylinder upon attaching operation.
Further, it is hard to detach a machine plate from the plate cylinder by lifting it up from the plate cylinder at the conclusion of a printing operation, because the presser plates are mounted so as to press the basic hole zone of the machine plate to the upper surface of clamp plates. Accordingly, Japanese Jikko Hei 6-11782 describes that at the conclusion of a printing operation the paper-made machine plate is detached from a plate cylinder by pulling the machine plate with a jerk in the direction of leaving the plate cylinder to break the machine plate at the basic holes. However, the way of breaking a machine plate upon detachment is unsuitable for plates hard to break, such as a plastic plate and an aluminum plate. This drawback of the attaching device cited above is not to be lightly disregarded.
Additionally, although the reusing of a plastic plate or an aluminum plate once used is desired in some cases, it is beyond the pale of the foregoing attaching device.
On the other hand, the machine plate attaching device described in Japanese Tokkai Hei 6-328669 is fitted with basic pins and, if desired, plates for pressing a machine plate (presser plates), and these fastening means are arranged between the clamp plate and the support of a PS plate lock-up device. Therefore, this device has an improvement over the aforementioned device with respect to the protection against injuries to workers. Further, this gazette discloses new embodiments wherein the basic pins are inserted in basic holes of a machine plate from the upper side of the machine plate towards the lower side thereof. These new embodiments can bring about an improvement in the aforementioned undesirable situation that the machine plate comes up and gets out of basic pins due to the reaction force the machine plate has when it is curved along the peripheral direction of a plate cylinder upon attaching operation, and thereby it is apt to slide off the plate cylinder. However, as the basic pins of the attaching device are designed so as to have almost the same diameter as the basic holes of a machine plate to be attached, it is difficult to insert the basic pins in the basic holes, resulting in the lowering of workability.
Furthermore, those machine plate attaching devices are obtained by fixing basic pins and other attachments to a conventional PS plate lock-up device fitted in the cut-off part of a printing drum in order to make improvements thereon, so that the resulting plate cylinders become expensive because of their complex structure, compared with conventional ones.